Catalytic hydrogenation is used for the production of bio-renewable chemicals, fuels, commodity chemicals, fine chemicals, and pharmaceuticals. Complexes of precious metals (e.g. Rh, Ir, Ru, Pd, or Pt) and ligands are used for catalytic hydrogenation. They exhibit high functional group tolerance, have long lifetimes and high activities, and may be used for hydrogenating carbonyl (C═O) groups, alkene (C═C) groups, alkyne (C≡C) groups, imine (C═N) groups, nitrile (C≡N) groups, and the like. Complexes of the precious metals rhodium, ruthenium, and iridium have been used for asymmetric catalytic hydrogenation. The development of base-metal-containing complexes (e.g. complexes of Mn, Fe, Co, Ni, Cu, and the like) for hydrogenation has lagged behind, perhaps because base metals tend to engage in one-electron or radical chemistry. Several complexes of iron, for example, were reported for catalytic hydrogenation of ketone groups or alkene groups, but the effectiveness of such complexes typically has been restricted to a particular class of substrate, and such complexes are often sensitive to water and also to compounds with oxygen-containing groups and/or nitrogen-containing groups.
Complexes of cobalt may be used for homogeneous hydrogenation of unsaturated compounds. HCo(CO)4, Co(H)(CO)(PnBu3)3, and related cobalt(I) complexes, for example, are used for the catalytic hydrogenation of alkenes and arenes at temperatures greater than 120° C. and pressures greater than 30 atm of hydrogen (“H2”). Complexes of cobalt with diiminopyridine ligands as well as the dinitrogen complex Co(H)(N2)(PPh3)3 are used for the catalytic hydrogenation of olefins at room temperature, and an asymmetric hydrogenation of substituted styrenes was recently developed. The cobalt(I) dihydrogen complex [(P(CH2CH2PPh2)3)Co(H2)]BPh4 was used for the catalytic hydrogenation of carbon dioxide (CO2) and bicarbonates to formic acid derivatives.
Reports are scarce for catalytic hydrogenation of aldehydes and ketones using complexes of cobalt. Aldehyde hydrogenation was reported as a side reaction in the catalytic hydroformylation of olefins at 185° C. with Co2(CO)8 and 300 atm of synthesis gas. The catalytic hydrogenation of aldehydes using the cobalt complex Co(II)(CO)(PnBu3)3 with 30 atm H2 has been reported, but reaction was complicated by a competing aldehyde decarbonylation reaction. A complex of cobalt and dioxime ligand has been used for the catalytic asymmetric hydrogenation of benzil, but the substrate scope was limited to 1,2-dicarbonyl compounds.
Nearly all prior examples of catalytic hydrogenation with complexes of cobalt have employed cobalt(I), i.e. cobalt in the +1 oxidation state, and most of these examples have been limited in substrate scope.
There are some examples of catalytic hydrogenation with complexes of nickel. A complex of nickel with a diphosphine-borane ligand was used for styrene hydrogenation at room temperature. The hydrogen activation mechanism was suggested to involve a cooperative metal-ligand interaction in which a nickel(0) complex accepts a proton and the boron on the ligand serves as a hydride acceptor.
Caulton et al. reported a complex of nickel of the formula [(PNP′)Ni]BArF4 (PNP′=−N(SiMe2CH2P(tBu)2)2) and proposed that it cleaved H2 heterolytically through a pathway having NiIV dihydride character. The PNP ligand is sometimes referred to in the art as a “pincer” ligand. This unusual example of H2 activation may involve an interaction between the nickel and the nitrogen of the pincer ligand shown in Scheme 1.
These types of interactions between the metal and ligand might also be involved in catalytic hydrogenation of polar multiple bonds using complexes of precious metals.